Non-aqueous electrolyte solutions containing a lithium salt are generally used in lithium ion batteries. The non-aqueous electrolyte solution comprises an aprotic polar solvent and a lithium salt dissolved therein. Carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate; lactones such as γ-butyrolactone, or ethers such as tetrahydrofuran are examples of the solvent. These organic solvents are generally volatile and inflammable. Therefore, their use can develop safety concern when the battery is overly charged and discharged or short-circuited. Also liquid electrolyte solutions must encounter some difficulties when sealing the battery in liquid tight manner. The electrolyte solution may be solidified into a gel but problems of volatilization and inflammability of the organic solvent as well as leakage of liquid electrolyte separated from gel still remain unsolved.
Recently a lithium ion battery has been proposed which utilizes a non-aqueous electrolyte comprising a room-temperature molten quaternary ammonium salt and a lithium salt dissolved therein. See, JP-A-10/92467, JP-A-10/265674, JP-A-11/92467 and JP-A-2002/11230. The molten salt is liquid at room temperature which is safe by virtue of its non-volatility and non-inflammability. The molten salt electrolyte may be solidified with a suitable polymer into a gel. However, the resulting gel possesses poor mechanical properties and often suffers from phase separation into liquid. Thus problems remain unsolved with respect to handling of electrolyte and battery design utilizing such a molten salt electrolyte.
JP-A-10/83821 and JP-A-2000/11753 propose a total solid polymer electrolyte prepared by polymerizing an ion conductive molten salt monomer having a vinyl group introduced into an imidazolium salt. This polymer electrolyte does not possess a sufficient mechanical strength.
Accordingly, a need exists for a polymer electrolyte which is safe and having both high ion conductivity and satisfactory mechanical strength.